ZnFe2O4@SiO2@L-lysine@SO3H: preparation, characterization, and its catalytic applications in the oxidation of sulfides and synthesis of Bis(pyrazolyl)methanes

Herein, we report the synthesis of ZnFe2O4@SiO2@L-lysine@SO3H as a green, novel magnetic nanocatalyst, containing the sulfuric acid catalytic sites on the surface of zinc ferrite as the catalytic support. The physical and chemical properties of raw and modified samples (ZnFe2O4@SiO2@L-lysine@SO3H) were characterized by TGA, EDX, PXRD, Map, and FTIR analyses. The prepared nanocatalyst has excellent catalytic activity in synthesizing the oxidation of sulfides to the sulfoxides and Synthesis of pyrazolyl (Bis(pyrazolyl)methane) derivatives under green conditions. This designed nanocatalyst offers several advantages including the use of inexpensive materials and high yield, simple procedure, and commercially available. The synthesized mesoporous nanocatalyst was recovered and reused in five continuous cycles without considerable change in its catalytic activity.

Preparation of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H The ZnFe 2 O 4 and ZnFe 2 O 4 @SiO 2 MNPs were prepared according to our previous methods respectively 28,29 .In the next step, the ZnFe 2 O 4 @SiO 2 (0.5 g) was dispersed in 60 mL DI (H 2 O) by sonication for 45 min.After vigorous stirring for 45 min, 1.5 mmol of L-lysine was added to the reaction mixture which was stirred at 60 °C degrees for 22 h.The product was separated by an external Neodymium magnet and washed with Ethanol and H 2 O and dried in an oven at 65 °C degrees to give ZnFe 2 O 4 @SiO 2 @L-lysine composite.Finally, to prepare ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H, the obtained ZnFe 2 O 4 @SiO 2 @L-lysine (1 gr) were added to the flask and dispersed ultrasonically for 30 min in dry hexane (35 mL).Chlorosulfonic acid (0.4 mL) was added dropwise to a cooled ice-bath dispersion of ZnFe 2 O 4 @SiO 2 @L-lysine for 35 min.Chlorosulfonic acid was slowly added to the reaction mixture at cool temperature.Then, the reaction mixture was subjected to continuous stirring for 24 h, while the residual HCl was eliminated by suction.The product was then separated from the reaction mixture by an external Neodymium magnet and washed several times with dried hexane.Finally, ZnFe 2 O 4 @SiO 2 @L-lysine@ SO 3 H was dried under vacuum at 60 °C (Fig. 1).

A general procedure for the oxidation of sulfides
A combination of sulfide (0.5 mmol) and H 2 O 2 (0.15 mL) containing ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H composite as catalyst (0.03 g) was stirred under solvent-free conditions at 25 °C.The progress of the reaction was monitored by TLC.Upon the completion of the reaction, the ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H were separated by a magnet, and the products were extracted by DI (H 2 O) and EtOAc.The organic phase was dried with Na 2 SO 4 (Fig. 3).

Catalyst characterization
Using FT-IR spectroscopy, the synthesis of zinc ferrite nanoparticles (ZF-NPS) was confirmed.The absorption band at 582 cm −1 is assigned to the stretching vibrations of the zinc-oxygen bond 30,31 .In Fig. 4a, the bending and stretching vibration of hydroxyl groups on the surface of the nanoparticles at 1655 and 3442 cm −1 are respectively assigned 29 .Figure 4b confirms the condensation reaction between hydroxyl groups of ZnFe 2 O 4 (MNPs) and the alkoxysilane molecules of tetraethyl orthosilicate (TEOS) as the first layer.Absorbed peaks at 3460 cm −1 were specified as hydroxide stretching vibration mode 30 .The two absorption peaks around 1103, and 606 cm −1 were indicated the presence of silicon-oxygen )Si-O-Si( asymmetric and symmetric stretching vibrations and bending vibration mode of silicon-oxygen (Si-O-Si), as well as a small peak around 1647 cm −1 , was assigned to hydroxide stretching vibration of Silicon-hydroxyl group and twisting vibration of adsorbed H-O-H in a silica shell 32 .In Fig. 4c, ZnFe 2 O 4 @SiO 2 @L-lysine, the bands in the range of 2912 to 3000mc −1 correspond to the bending vibration of CH 2 confirming the attachment of L-lysine molecules to the surface.Then, the presence of broad band at 2500-3700 cm −1 in FTIR spectra of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H (Fig. 4d) confirms the successful functionalization of ZnFe 2 O 4 @SiO 2 with the SO 3 H groups 28 .
The PXRD analysis for the used catalyst was provided and the results were compared to the fresh catalyst, which shows high stability of the prepared catalyst under optimized reaction conditions (Fig. 6).
Figure 7 shows the TGA curves for ZnFe 2 O 4 MNPs, ZnFe 2 O 4 @SiO 2 , ZnFe 2 O 4 @SiO 2 @L-lysine, and ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H.In all samples, the first step of weight loss (below 200 °C) is owing to the removal of physically absorbed water and organic solvents (Fig. 7a-d).The decomposition of the organic layer on ZnFe 2 O 4 has occurred in the TGA curve of the catalyst from 200 to 500 °C.Meanwhile, weight loss of about 2% and 8% from 200 to 500 °C occurred for SiO 2 and L-lysine, respectively (Fig. 7b and c). Figure 7d illustrates two weight loss steps in the TGA curve of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H.The first weight loss (10%) between 25 and 250 °C is occurred due to the removal of adsorbed moisture and organic solvents.The next weight loss (50%) from  www.nature.com/scientificreports/250 to 600 °C is due to the degradation of organic moieties and the chemisorbed sulfuric acid groups on the surface of the ZnFe 2 O 4 core.Based on the results of the TGA-DSC curve, the well grafting of organic groups on the ZnFe 2 O 4 magnetic nanoparticles is verified 34 .The distribution, size, surface morphology, particle shape, and fundamental physical properties of ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H nanoparticles were investigated using the SEM technique (Fig. 8).The ZnFe 2 O 4 @SiO 2 @Llysine@SO 3 H composite is spherical with an almost homogenous size distribution.In addition, the SEM image shows that the size of the nanoparticles is about ≈ 81 nm (Fig. 8).
In another investigation, EDX analysis confirmed the presence of Zn, C, O, Si, N, Fe, and S elements in the synthesized ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H.As shown in Fig. 9, the presence of Si species confirmed the successful bonding of the SiO 2 shell on the ZnFe 2 O 4 catalytic support.The high purity of the synthesized nanocatalyst was confirmed by these observations.It can be concluded that the target catalyst has been successfully synthesized according to this EDX spectrum (Fig. 9).
The X-ray mapping of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H shows the scattering of elements in the ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H (Fig. 10).This analysis confirms the presence of Si, Fe, S, N, C, Zn, and O elements in the synthesized nanoparticle with a suitable and homogeneous dispersity throughout the ZnFe 2 O 4 surface.
Using TEM images, the core-shell structure of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H cubic nanoparticles was investigated.From Fig. 11, we can see the cubic nanoparticles of the ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H composites.The TEM micrograph showed agglomeration of many ultrafine cubic particles which display gray magnetite (ZnFe 2 O 4 ) cores surrounded by a SiO 2 @L-lysine@SO 3 H shell.It is very interesting that the TEM image again verifies the yolk-shell microstructure in ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H, and it is clear that dense silica layers and L-lysine@SO 3 H were formed around ZnFe 2 O 4 nanocores (Fig. 11).
The surface area and size distribution of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H acid is studied by N 2 adsorption-desorption isotherms analysis.Regarding the N 2 adsorption-desorption isotherms technique, the obtained surface area of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H is 6.42 (m 2 /g) based on the BET method.Also, the total pore volume and average pore diametere of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H are obtained by the BET technique and the values are 0.07 cm 3 g −1 , and 44 nm, respectively (Fig. 12).
Using the back titration method, the acid strength of the synthesized catalyst, that is, the surface density of SO 3 H groups, was investigated and determined.First, 0.1 g of synthesized catalyst was added to the 50 mL water and stirred for 1 h, then 10 mL NaOH (0.1 N) was added to the mixture and was stirred as long as the pH did not change the as-synthesized catalyst was separated using an external magnet.Then, two drops of phenolphthalein were added to the mixture and were tittered with 1.9 mL HCl (0.1 N).Thus 1 g of catalyst has 8.1 mmol of the acidic groups.
The magnetic behavior of ZnFe 2 O 4 (a) and ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H (b) composite was investigated with the vibrating sample magnetometer (VSM) (Fig. 13).The ZnFe 2 O 4 nanoparticles exhibited almost zero coercivity and remanence with no hysteresis loop, approving the high permeability in magnetization and good magnetic responsiveness.Magnetic measurements showed saturation magnetization values of 41 and 22 emu/g for ZnFe 2 O 4 and ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H complex nanocomposite, respectively.The results showed that the magnetization of ZnFe 2 O 4 decreases after the coating of L-lysine@SO 3 H on its surface, indicating the successful immobilization of the L-lysine@SO 3 H on ZnFe 2 O 4 .

Catalytic study
Checking catalytic activity of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H for the synthesis of pyrazolyl In the next step, after the successful synthesis and characterization of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H, its catalytic activity was considered for the synthesis of pyrazolyl derivatives and oxidation of sulfides.
In early research to obtain optimal reaction conditions, after structural characterization of the prepared nanocatalyst (ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H), its catalytic activity was investigated in the synthesis of pyrazolyl (Table 1).The reaction between benzaldehyde (1 mmol), phenylhydrazine (2 mmol), and ethyl acetoacetate (2 mmol), was selected as the model reaction, and the influence of various parameters including amounts of catalyst, reaction temperature, and solvent were examined.The model reaction did not take place in the absence of the ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H.After optimizing the catalyst's amount, the effect of temperatures and several solvents was checked.The best results were obtained in solvent-free conditions using 0.03 g ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H at 80 °C.
After determining the optimal conditions, to identify the performance and generality of ZnFe 2 O 4 @SiO 2 @Llysine@SO 3 H, the synthesis of diverse derivatives such as pyrazolyl was tested by various arylaldehydes (Table 2).As can be observed in this table, all arylaldehydes worked well in the reaction and it was observed that the synthesis of pyrazolyl in the presence of this catalyst afforded excellent yields with short reaction times.
The mechanism for the synthesis of pyrazolyl in the presence of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H has been depicted in Fig. 14.At the beginning of the reaction, ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H composite activates C=O groups in the ethyl acetoacetate, and then phenylhydrazine attacks the C=O groups to afford pyrazolone 1 and was further rearranged into tautomer 2. Next, a Knoevenagel-type reaction takes place between activated aldehydes and tautomer 2 followed by the liberation of an H 2 O molecule to form intermediate 3.Then, a Michael addition reaction between intermediate 3 and tautomer 2 is facilitated to form intermediate 4. In the final step, the corresponding products are formed by tautomerization and aromatization of intermediate 4 35 .
After characterization of the synthesized heterogeneous ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H nanocatalyst was examined in the oxidation of sulfide to understand the catalytic activity of the prepared material.First, the reaction of the Ph-S-Me with H 2 O 2 was selected as a model reaction and carried out in the presence of ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H Hat different conditions, including different temperatures and amounts of nanocatalyst, and the results showed that the catalyst showed high activity in solvent-free conditions at 25 °C at 120 min.In the next step, the effect of different solvents (EtOAc, n-Hexane, Ethanol, H 2 O) and also the conditions without solvent were investigated.It should be noted that in solvent-free conditions, the best yield was obtained in 120 min.The study of the amount of catalyst showed that the 0.03 g of nanocatalyst gave a high yield of product (Table 3, entries 1-5).The oxidation didn't occur in the absence of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H even after 4 h (Table 3).In the next step, after the completion of optimization, the catalytic activity of ZnFe 2 O 4 @SiO 2 @L-lysine@ SO 3 H in the oxidation of a wide range of sulfide derivatives was examined under the optimized conditions.It is necessary to mention that all sulfoxides were produced with high yields, which showed the excellent catalytic activity of the synthesized nanoparticles (Table 4).This catalytic system is a suitable method in terms of the efficiency of conditions.
The proposed mechanism for the oxidation of sulfide to the corresponding sulfoxide is shown in Fig. 15.The efficiency of the oxidation can be explained by the interaction between the ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H and H 2 O 2 .The OH moiety of the ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H forms a strong hydrogen bond with H 2 O 2 and increases the electrophilic ability of a peroxy oxygen atom of H 2 O 2 .In these reaction conditions hydrogen bonding may be assisting in controlling the chemoselectivity, because the hydrogen bond between the catalyst and the oxygen of the sulfoxides could decrease the nucleophilicity of the sulfur atom of the sulfoxides and prevent further oxidation of the sulfoxides.One explanation for this transformation is the in-situ formation of peroxy acid using the reaction of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H with Hydrogen peroxide, followed by the oxygen transfer to the organic substrate (Fig. 15a).Another explanation is that ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H acts as protic acid, which polarizes the oxygen-oxygen bond in hydrogen peroxide to produce the reactive oxygen transfer agent (Fig. 15b) 40 .Table 1.Optimization of reaction conditions for the synthesis of 4,4'-(phenylmethylene)bis(3-methyl-1phenyl-1H-pyrazol-5-ol) in the presence of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H as a catalyst.a Reaction was performed in the presence of ZnFe 2 O 4 @SiO 2 .b Reaction was performed in the presence of ZnFe 2 O 4 @SiO 2 @Llysine.d Reaction was performed in the presence of L-lysine.

Hot filtration
In this part, with optimal reaction conditions in hand, to confirm the heterogeneous nature of the ZnFe 2 O 4 @ SiO 2 @L-lysine@SO 3 H in the synthesis of pyrazolyl compounds hot filtration experiment was performed using benzaldehyde as a model reaction.At the half time of reaction, the corresponding product was obtained in 55% of the yield.Next, when the reaction mixture was run in another half-time in the absence of nanocatalyst, the reaction afforded no augmentation in its yield.It can be concluded from this point that the catalyst can be considered a true heterogeneous nanocatalyst.Moreover, the stability of the L-lysine@SO 3 H complex on the surface of ZnFe 2 O 4 confirms the heterogeneous nature of the as-prepared nanocatalyst.
Reusability of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H The recoverability of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H catalyst was investigated for oxidation of sulfides (series 1) and Synthesis of pyrazolyl (series 2) derivatives.In this study, the recovery of the nanocatalyst from the reaction mixture was successfully carried out, which could be easily separated with a neodymium magnet and washed several times with EtOAc and DI (H 2 O).Then the recovered nanocatalyst was used in the next run.The results showed that recycled catalysts can be employed at both of the reactions up to five times, with insignificant loss of catalyst activity (Fig. 16).

Comparison of the catalyst
The comparative study of different catalytic for the synthesis of pyrazolyl derivatives (Table 5), with several previously reported methods, is presented.In the present research, the products were obtained in higher yields over faster times in the presence of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H.In addition, this catalyst is environmentally friendly and has several advantages in terms of sustainability, price, separation, and non-toxicity.

Conclusions
In this research project, we have successfully synthesized ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H nanoparticles as an effective and recoverable nanocatalyst.Wide active surface area, reusability, suitable stability, excellent heterogeneity, and substantial magnetic behavior have distinguished this catalytic system as an instrumental tool for the synthesis of organic compounds.This research reported a novel route for the synthesis of an extensive range of synthesis of pyrazolyl derivatives and oxidation of sulfides with high yields and purity.The wondrous features of this protocol are novelty, no use of harmful solvents, simple synthesis procedure, short reaction time, facile filtration, and reusability.In addition, the as-synthesized magnetic nanocatalyst could be separated easily using a external magnet and reused several times without significant loss of its catalytic activity.Table 5.Comparison results of ZnFe 2 O 4 @SiO 2 @L-lysine@SO 3 H with other catalysts in the synthesis of pyrazolyl.

Figure 15 .Figure 16 .
Figure 15.Possible mechanism for the oxidation of sulfide.
a Isolated yield.